1. Field of the Invention
The present invention relates to implantation of ions into silicon wafers and to the measurement of ion dose during an ion implantation process.
2. Description of the Related Art
In a conventional ion implanter dose control system, the main ion beam is sampled either periodically or continuously during wafer implantation to provide a measure of the ion particle current (particles/sec.).
The ion beam produced by an ion implanter is subject to both long and short term variations in intensity. Since the beam is generally much smaller than the wafer target, providing a spatially uniform dose on the wafer requires that either the beam is translated across the wafer or that the wafer is translated through the beam. It is common practice to modulate the translational velocity by a factor derived from the beam particle current to compensate for variations in beam current during an implant.
Before the translational velocity correction can be determined, the main (or implant) beam current must be known. There are many ways of satisfying this requirement. Two of the commonest are:
1) Multiplexing the beam (either spatially or temporally) to provide a sample of its intensity during the implant.
2) Collecting and measuring the current striking the wafer directly.
Both of these techniques suffer from the disadvantage that they measure the beam current in the proximity of the wafer. The wafer may induce local environmental perturbations which adversely affect the accuracy of the beam current measurement, resulting in dose errors. For example, if the wafer outgasses under the impact of the beam, the resultant vacuum degradation will cause undesired charge exchange of the beam, leading to an error in the beam current measurement.
The invention provides a novel means of measuring and ensuring that silicon wafers receive the correct ion dose (particles/cm2) during an ion implantation process.
In a conventional ion implanter dose control system, the main ion beam is sampled periodically during wafer implantation to provide an estimate of the ion particle current (particles/sec.).
In the present invention, the particle current of the main ion beam is estimated by measuring the ion current of an alternate (unused, mass resolved) ion beam which possesses a charge state which is undesirable for wafer implantation. Since the alternate (or monitor) beam is not used for wafer implantation, it can be monitored continuously and in a region of stable vacuum quality where ion currents can be measured most accurately. The ion current in the monitor beam is proportional to the ion current in the main beam and their ratio can be measured between wafer implantation batches. Therefore, by measuring the ion current of the monitor beam, the ion current of the main beam can be inferred. This method provides greater accuracy, especially in the presence of hydrogen outgassing from photoresist coated wafers, since the measurement is performed upstream from the poor vacuum of the wafer process chamber.